1. Field of the Invention
The invention relates in general to an image scanning apparatus, and more particularly to an apparatus, which precisely and effectively positions and determines the scan-reference point.
2. Description of the Related Art
The use of image scanners has become more and more popular along with the coming of multimedia age. Meanwhile, the users' expectations of the quality and speed of image scanners are getting higher and higher. The carriage of an image scanner is normally driven using a step motor. After having finished scanning, it is not guaranteed that the carriage will return to exactly the same home position. If the carriage of an image scanner used to fetch images can be effectively and precisely shifted from home position to a document scan-starting point before scanning, no parts of the to-be-scanned documents will be missed out, nor will there be any non-document images scanned. Thus, the quality of document scanning can be improved. Conventionally, before image scanning is started, a ‘scan-reference point’ which has been predefined before the scanner leaves factory must be located first. Given the coordinates of the scan-reference point already known, the position of the scan-starting point will be located according to the ‘vector relationship’ between the scan-reference point and the scan-starting point, wherein the vector relationship has been defined before the scanner leaves factory. Conventional methods for positioning a starting point for image scanning are as follows:
(a) Referring to FIG. 1, a method using a corner of a reflection area 12 on a calibration sheet 10 as a reference point Q to define the position of the scan-starting point is disclosed in Taiwanese publication patent no. 147499. The relative position between the reference point Q and scan-starting point P has already been defined before the scanner leaves factory. To locate the position of the reference point Q, the carriage 11 only needs to move in the direction of Y-axis differentiating color changes between the reflection area 12 and the calibration sheet 10. The position of the scan-starting point P will thus be located accordingly; and the carriage 11 will start scanning after having returned to the scan-starting point P.
(b) Referring to FIG. 2, a method using an existing mark 22 on a calibration sheet 20 to define the position of the scan-starting point is disclosed in Taiwanese publication patent no. 338868. When the carriage 21 stops on any scan lines of the mark 22, two reference points Q1 and Q2 are chosen from the mark 22; the position of the scan-starting point P will be located according to the coordinates of the two reference points, the predefined function relation of mark 22, and a predefined length between a predefined reference point Q on the mark 22 and the scan-starting point P. After that, scanning proceeds.
However, the conventional method will result in at least the following three types of errors when printing a reflection area and a mark and installing a calibration sheet:
First, the error would occur when printing a reflection area or mark onto a calibration sheet.
Second, the error of cutting calibration sheet would occur. That is, when cutting a large calibration sheet into smaller ones after having finished the printing of reflection areas or marks onto a large calibration sheet, the cutting process could not produce patterns whose specifications of sizes and distances would match their original designs completely.
Third, the error of pasting would occur. That is, when pasting a calibration sheet onto a scanning apparatus, the position onto which the calibration sheet is pasted could not match with the position and vector relation of its original design completely.
When scanning images, a carriage's movements on the X-axis and Y-axis are measured by picture elements. Taking an ordinary scanner whose resolution is 600 dpi for instance, there are 600 picture elements per inch. For a scanner requiring a high standard of precision, the above-mentioned errors are not only unbearable but also unavoidable.